Add support for Django 2.1

[ThePumpingLemma]

Description

Add Django 2.1 to the testing matrix and documentation.

Motivation and Context

We want to ensure that this library works on Django 2.1.

How Has This Been Tested?

Tested on TravisCI.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-io]

Codecov Report

Merging #418 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #418   +/-   ##
=======================================
  Coverage   97.32%   97.32%           
=======================================
  Files          14       14           
  Lines         672      672           
  Branches       93       93           
=======================================
  Hits          654      654           
  Misses          9        9           
  Partials        9        9

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